The disclosed subject matter relates generally to communication systems and, more particularly, to a method and apparatus for metallic line testing of a subscriber line.
Over the last few years, the demand for high speed communication applications has exploded. The Internet, for example, has grown at astronomical rates over the past several years. A significant number of new Internet subscribers connect from a home or small office using a personal computer (PC).
Digital subscriber line (xDSL) technologies have been developed to provide high-speed data transmission from the service provider (e.g., the central office) to the customer premise over the existing twisted-pair copper wiring conventionally used for telephone service. Such xDSL technologies leverage modem technology to increase the data transfer bandwidth of the twisted-pair copper wiring. Typically, xDSL modems are provided on the ends of the subscriber line copper wiring to communicate between the CO and the customer's premise. The manner in which the two modems communicate is established by the particular xDSL approach used. Because the existing telephone wire is used, xDSL technologies data signals are typically transferred out-of band with the voice band signals. Because different frequencies are used for the voice band and the data band, voice and data information can be concurrently transferred over the twisted-pair copper line. In a typical xDSL example, voice information may be carried in frequency bands below 4 kHz with data being carried in frequencies above the voice band, typically from 50 kHz to 30 MHz.
More recently, service providers have increased data bandwidth by installing fiber optic cabling between the central office and a multi-service access platform (MSAP) closer to the customer. A particular MASP may interface with a bundle of twisted pairs to service a relatively small number of customer premise connections. This approach shortens the length of the copper pair between the CO interface at the MSAP and the customer, thereby allowing increased DSL data rates.
One difficulty arising from an optical connection between the central office and the MSAP lies in the ability to test the metallic lines servicing the customer premises. Previously, test equipment for conducting the metallic line testing (MELT) resided at the central office. While an MSAP equipped with both POTS and xDSL cards can provide the adequate MELT functions, there are some deployments with only an xDSL card. Such systems, cannot provide the MELT function needed to appropriately test the line. Now that the connections leaving the central office are increasingly optical, the test equipment must be located at the MSAP rather than the central office. Another difficulty arising from the optical central office arrangement is that copper twisted pairs must typically be supplied with a wetting current to prevent corrosion. Again, in instances which the MSAP contains both POTS and xDSL, such wetting current functions would be supplied by the POTS card, however, in a naked xDSL scenario, an outside source is needed to inject the wetting current. Because the central office cannot supply a wetting current through its optical connection, that function must also be distributed.
Typical DSL implementations employ an isolation transformer between the central office connection (i.e., which may be at the MSAP) and the customer premise equipment (CPE). One technique for providing metallic line testing capability involves using a relay on the line side of the isolation transformer to switch in a metallic test unit to test the copper line. A splitter is also used for impedance matching to switch from the DSL impedance (e.g., 100 ohms) to the subscriber line impedance used for testing (e.g., 600 ohms for U.S. and 900 ohms for Europe). During the metallic line testing, the DSL line card is disabled. This testing arrangement gives rise to several issues. First, DSL service for a user must be interrupted during the testing. Second, the relay and splitter network adds impedance thereby affecting the characteristics of the line, and also, when leakage is measured on the line, leakage from the relay and splitter network contributes to the overall leakage measurement.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.